Re: [PATCH v5 00/15] x86: Add support for Clang CFI

Peter Zijlstra
2021-10-27T10:37:18Z

On Wed, Oct 27, 2021 at 10:02:56AM +0000, David Laight wrote:
> From: Peter Zijlstra
> > Sent: 26 October 2021 21:16
> >
> > On Wed, Oct 13, 2021 at 11:16:43AM -0700, Sami Tolvanen wrote:
> > > This series adds support for Clang's Control-Flow Integrity (CFI)
> > > checking to x86_64. With CFI, the compiler injects a runtime
> > > check before each indirect function call to ensure the target is
> > > a valid function with the correct static type. This restricts
> > > possible call targets and makes it more difficult for an attacker
> > > to exploit bugs that allow the modification of stored function
> > > pointers. For more details, see:
> > >
> > > https://clang.llvm.org/docs/ControlFlowIntegrity.html
> >
> > So, if I understand this right, the compiler emits, for every function
> > two things: 1) the actual funcion and 2) a jump-table entry.
> >
> > Then, every time the address of a function is taken, 2) is given instead
> > of the expected 1), right?
> >
> > But how does this work with things like static_call(), which we give a
> > function address (now a jump-table entry) and use that to write direct
> > call instructions?
> >
> > Should not this jump-table thingy get converted to an actual function
> > address somewhere around arch_static_call_transform() ? This also seems
> > relevant for arm64 (which already has CLANG_CFI supported) given:
> >
> > https://lkml.kernel.org/r/20211025122102.46089-3-frederic@kernel.org
> >
> > Or am I still not understanding this CFI thing?
>
> From what I remember the compiler adds code prior to every jump indirect
> to check that the function address is in the list of valid functions
> (with a suitable prototype - or some similar check).

It definitely mucks about with the address too; see here:

https://lkml.kernel.org/r/YW6a67fGzM2AyHot@hirez.programming.kicks-ass.net

I'm thinking static_call() wants the real actual function address before
it writes instructions.